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Modelling of the spectral energy distribution of Fornax A: leptonic and hadronic production of high-energy emission from the radio lobeshttp://hdl.handle.net/2289/6184
Title: Modelling of the spectral energy distribution of Fornax A: leptonic and hadronic production of high-energy emission from the radio lobes<br/><br/>Authors: McKinley, B.; Yang, R.; Deshpande, A.A.; Udaya Shankar, N.; Srivani, K.S.; Subrahmanyan, Ravi; +40 Co authors<br/><br/>Abstract: We present new low-frequency observations of the nearby radio galaxy Fornax A at 154 MHz with the Murchison Widefield Array, microwave flux-density measurements obtained from WMAP and Planck data, and γ-ray flux densities obtained from Fermi data. We also compile a comprehensive list of previously published images and flux-density measurements at radio, microwave and X-ray energies. A detailed analysis of the spectrum of Fornax A between 154 and 1510 MHz reveals that both radio lobes have a similar spatially averaged spectral index, and that there exists a steep-spectrum bridge of diffuse emission between the lobes. Taking the spectral index of both lobes to be the same, we model the spectral energy distribution of Fornax A across an energy range spanning 18 orders of magnitude, to investigate the origin of the X-ray and γ-ray emission. A standard leptonic model for the production of both the X-rays and γ-rays by inverse-Compton scattering does not fit the multiwavelength observations. Our results best support a scenario where the X-rays are produced by inverse-Compton scattering and the γ-rays are produced primarily by hadronic processes confined to the filamentary structures of the Fornax A lobes.<br/><br/>Description: Open AccessLimits on low-frequency radio emission from southern exoplanets with the Murchison widefield arrayhttp://hdl.handle.net/2289/6183
Title: Limits on low-frequency radio emission from southern exoplanets with the Murchison widefield array<br/><br/>Authors: Murphy, Tara; Deshpande, A.A.; Prabhu, T.; Udaya Shankar, N.; Srivani, K.S.; Subrahmanyan, Ravi; +25 Co-authors<br/><br/>Abstract: We present the results of a survey for low-frequency radio emission from 17 known exoplanetary systems with the Murchison Widefield Array. This sample includes 13 systems that have not previously been targeted with radio observations. We detected no radio emission at 154 MHz, and put 3σ upper limits in the range 15.2–112.5 mJy on this emission. We also searched for circularly polarized emission and made no detections, obtaining 3σ upper limits in the range 3.4–49.9 mJy. These are comparable with the best low-frequency radio limits in the existing literature and translate to luminosity limits of between 1.2 × 1014 and 1.4 × 1017 W if the emission is assumed to be 100 per cent circularly polarized. These are the first results from a larger program to systematically search for exoplanetary emission with the MWA.<br/><br/>Description: Open AccessPossible proton synchrotron origin of X-ray and gamma-ray emission in large-scale jet of 3C 273http://hdl.handle.net/2289/6182
Title: Possible proton synchrotron origin of X-ray and gamma-ray emission in large-scale jet of 3C 273<br/><br/>Authors: Kundu, Esha; Gupta, Nayantara<br/><br/>Abstract: The large-scale jet of quasar 3C 273 has been observed in radio to gamma-ray frequencies. Earlier the X-ray emission from knot A of this jet has been explained with inverse Compton scattering of the cosmic microwave background radiations by the shock accelerated relativistic electrons in the jet. More recently it has been shown that this mechanism overproduces the gamma-ray flux at GeV energy and violates the observational results from Fermi LAT. We have considered the synchrotron emission from a broken power-law spectrum of accelerated protons in the jet to explain the observed X-ray to gamma-ray flux from knot A. The two scenarios discussed in our work are (i) magnetic field is high, synchrotron energy loss time of the protons is shorter than their escape time from the knot region and the age of the jet and (ii) their escape time is shorter than their synchrotron energy loss time and the age of the jet. These scenarios can explain the observed photon spectrum well for moderate values of Doppler factor. The required jet luminosity is high ∼1046 erg s−1 in the first scenario and moderate ∼1045 erg s−1 in the second, which makes the second scenario more favourable.<br/><br/>Description: Open AccessLarge-scale dynamo action due to α alpha fluctuations in a linear shear flowhttp://hdl.handle.net/2289/6170
Title: Large-scale dynamo action due to α alpha fluctuations in a linear shear flow<br/><br/>Authors: Sridhar, S.; Singh, Nishant K.<br/><br/>Abstract: We present a model of large-scale dynamo action in a shear flow that has stochastic, zero-mean fluctuations of the α parameter. This is based on a minimal extension of the Kraichnan–Moffatt model, to include a background linear shear and Galilean-invariant α-statistics. Using the first-order smoothing approximation we derive a linear integro-differential equation for the large-scale magnetic field, which is non-perturbative in the shearing rate S , and the α-correlation time τα . The white-noise case, τα = 0 , is solved exactly, and it is concluded that the necessary condition for dynamo action is identical to the Kraichnan–Moffatt model without shear; this is because white-noise does not allow for memory effects, whereas shear needs time to act. To explore memory effects we reduce the integro-differential equation to a partial differential equation, valid for slowly varying fields when τα is small but non-zero. Seeking exponential modal solutions, we solve the modal dispersion relation and obtain an explicit expression for the growth rate as a function of the six independent parameters of the problem. A non-zero τα gives rise to new physical scales, and dynamo action is completely different from the white-noise case; e.g. even weak α fluctuations can give rise to a dynamo. We argue that, at any wavenumber, both Moffatt drift and Shear always contribute to increasing the growth rate. Two examples are presented: (a) a Moffatt drift dynamo in the absence of shear and (b) a Shear dynamo in the absence of Moffatt drift.<br/><br/>Description: Open Access